Twin and Earth CPC

[Cookie]

IEC 60364-5-54 states that phase conductors 16mm2 and smaller require a CPC of the same size, over 16 to 35mm2 a 16mm2 CPC and over 35mm2 a half size CPC is required.

My questions is why am I seeing catalogs with harmonized twin and earth with reduced size CPCs over 2.5mm2? And why doesn't the half size rule kick in when wire is over 4mm2 instead of 16mm2?

 

[Julie.]

Perhaps that is the issue... Here are the requested pages:

It is explained in 543.1.1, perform the calculation in 543.1.2 OR use the table 54.3

 

[Cookie]

Germany and other European country's were well ahead of us in adopting MCB and RCDs

We are now playing catch up, Europe is now writing the standards.

Yes, but I'd argue thats because over their hodge podge of plugs which do not always mate with an earth.

 

[pc1966]

Yes, but I'd argue thats because over their hodge podge of plugs which do not always mate with an earth.

It is not just the "will it or won't it" function of the earth pin or side strip, etc, but generally they have non-polarised plug so you can swap N & L, and also many EU supplies are TT so needed the RCD incomer at least for any chance for acceptable fault clearance.

But you are also right that the UK (and some other countries) biggest blind spot is using TN-C-S and the resulting risk from a PME fault. As mentioned before by @davesparks what they should have done is insist on every new property on PME having its own earth rod(s) as well. Sure one rod is not going to help much with a PME fault, but having many, many more rods would be more fault tolerant than a handful of supply rods.

 

[Ian1981]

They’ve used PME to save money on cable but ultimately it will backfire as the network breaks down and the money they supposedly saved , is spent on repairs, or shove the onus onto the consumer and make them come up with a solution to negate an open PEN fault.

 

[pc1966]

They’ve used PME to save money on cable but ultimately it will backfire as the network breaks down and the money they supposedly saved , is spent on repairs

I don't think TN-C-S is any more likely to break down than TN-S. I it just the consequences that are more serious!

 

[Ian1981]

I don't think TN-C-S is any more likely to break down than TN-S. I it just the consequences that are more serious!

If they stuck to TNS then the open PEN consequences never exist.
Protective bonding conductors could be smaller etc, EV chargers won’t require expensive devices to protect against open PEN faults , no earth electrodes to install and would be safer.
I’m not an engineer so I maybe looking at this one sided but that’s what I think anyway.

 

[Cookie]

It is explained in 543.1.1, perform the calculation in 543.1.2 OR use the table 54.3

The way I interpret it you must perform a calculation

It is not just the "will it or won't it" function of the earth pin or side strip, etc, but generally they have non-polarised plug so you can swap N & L, and also many EU supplies are TT so needed the RCD incomer at least for any chance for acceptable fault clearance.

But you are also right that the UK (and some other countries) biggest blind spot is using TN-C-S and the resulting risk from a PME fault. As mentioned before by @davesparks what they should have done is insist on every new property on PME having its own earth rod(s) as well. Sure one rod is not going to help much with a PME fault, but having many, many more rods would be more fault tolerant than a handful of supply rods.

I agree, but I don't think polarity makes much of a difference. My understanding is the EU sockets are designed such reverse polarity will not shock you.

Regarding earth rods- do you really want the earth becoming an even larger conductor? We are using the planet as an experiment, one with no "reset" or "stop" button if something goes wrong.

In the US stray voltage cases have resulted in a steady stream of law suits against utilities.

 

[Julie.]

The way I interpret it you must perform a calculation

No it's very clear you must EITHER perform the calculation in 543.1.2 OR select from the table 54.3

it is not ambigus

 

[Lucien Nunes]

From an electronic engineering standpoint, TN-C / TN-C-S is fundamentally flawed. You cannot use a conductor to simultaneously establish an equipotential and pass a current, unless it has zero resistance. My opinion is coloured by the fact that I design studio-grade analogue audio electronics as part of the day job, for which the resulting circulating currents and CPC / true earth voltage gradients can be a serious nuisance that would in theory be almost eliminated with pure TN-S

From an electrical standpoint, I can accept that with suitable engineering standards adhered-to rigidly, the additional risk of open PEN faults could be mitigated so as to be an insignificant contributor to the total risk arising from the use of electrical power.

But returning to the subject of CPC size, has anyone here done any practical experiments to satisfy themselves of the validity of the adiabatic limit? I have, years ago, using a very large battery bank, and the results were as expected and unremarkable.

 

[Cookie]

No it's very clear you must EITHER perform the calculation in 543.1.2 OR select from the table 54.3

it is not ambigus

Then how could a reduced size earth in T&E be compliant unless the installer calculates it?

 

[Cookie]

From an electronic engineering standpoint, TN-C / TN-C-S is fundamentally flawed. You cannot use a conductor to simultaneously establish an equipotential and pass a current, unless it has zero resistance. My opinion is coloured by the fact that I design studio-grade analogue audio electronics as part of the day job, for which the resulting circulating currents and CPC / true earth voltage gradients can be a serious nuisance that would in theory be almost eliminated with pure TN-S

You. I like you

Audio noise is a big problem in the US in that not only is TN-C-S the only major earthing means, but also the fact older buildings are riddle with standing neutral to ground faults on top of 240 volt appliances which earth through the neutral. Lack of RCD means nothing will detect these faults or wiring errors.

The NEC allows for isolated grounding sockets which allow the user to run an insulated CPC all the way back to the service or origin of power supply. It works well until the metal chassis losses its isolation which is inevitable in something like a studio.

The other issue that few recognize are high magnetic fields which I personally do not believe people should be exposed to when they can entirely be mitigated through RCDs and TT/TN-S earthing.

Personally if I had a choice I would use a 138/240Y system. Connect everything phase to phase and have the neutral point just for earthing purposes. Any fault will trip a breaker. Half the difficulty would disappear. Lots of debates spared.

https://imgur.com/CfBnL39

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From an electrical standpoint, I can accept that with suitable engineering standards adhered-to rigidly, the additional risk of open PEN faults could be mitigated so as to be an insignificant contributor to the total risk arising from the use of electrical power.

That is until a fire starts:

Open Neutral

But returning to the subject of CPC size, has anyone here done any practical experiments to satisfy themselves of the validity of the adiabatic limit? I have, years ago, using a very large battery bank, and the results were as expected and unremarkable.

The NEC's Table 250.122 is living proof the adiabatic method might actually be ultra conservative.

 

[Julie.]

Then how could a reduced size earth in T&E be compliant unless the installer calculates it?

Because it is a standard circuit and it has already been calculated - the whole point of standard circuits is that all the factors have been calculated!

You could calculate it again, but why?

If I have a circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA

Knowing this - I now calculate it again - Why???

OK, so k=115 the let through from 3kA based on a 16A mcb is 1.98kA, and the trip time is 0.003s (it's well above the inst trip into current limiting)

This works out as SQRT(1980x1980x0.003) / 115 = 0.94mm^2

Isn't that odd, you use the figures provided by the IET as acceptable - and when you calculate it - it actually works out!

Now I go to a different site, and need another circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA

Do I calculate it again?

 

[Cookie]

Because it is a standard circuit and it has already been calculated - the whole point of standard circuits is that all the factors have been calculated!

You could calculate it again, but why?

If I have a circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA

Knowing this - I now calculate it again - Why???

OK, so k=115 the let through from 3kA based on a 16A mcb is 1.98kA, and the trip time is 0.003s (it's well above the inst trip into current limiting)

This works out as SQRT(1980x1980x0.003) / 115 = 0.94mm^2

Isn't that odd, you use the figures provided by the IET as acceptable - and when you calculate it - it actually works out!

Now I go to a different site, and need another circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA

Do I calculate it again?

Ill take your word for it. That if it means if means loop impedance requirements are met than the CPC will always be greater than what the adiabatic method would calculate out to be when dealing with T&E.

 

[Cookie]

In reality few electricians have to use the adiabatic equation (though they should know how to) as the IET's On Site Guide has some useful tables that incorporate the information for circuit design. For example this table is for BS88 fuses and shows the maximum Zs values for different CPC sizes:
View attachment 58753
For example, if you have 10mm T&E cable with a 4mm CPC used as a sub-main feed so you could allow 5s disconnection, you might have a 63A fuse for short circuit protection only, and then the downstream DB can use a mix of MCBs up to 32A in order to provide overload protection with a reasonable chance of selectivity. Looking at the above table you see you max measured Zs is 0.49 ohms, so your final test at the nice new DB would be to confirm this is met.

Also you see the value is 0.62 ohms in all the larger CPC sizes - they are time-limited for the fuse action, where as at 4mm it is adiabatically limited (hence lower Zs for a shorter fault disconnection time).

This helps- thank you!

OK, so k=115 the let through from 3kA based on a 16A mcb is 1.98kA, and the trip time is 0.003s (it's well above the inst trip into current limiting)

On a side note. Instantaneous in current liming like a fuse? As I understand it a fuse begins to melt as soon as it gets hot, while a solenoid must saturate, pull in, and then wait to unlatch with an arc which takes time to extinguish.

I know that RK low peak fuses tend to reduce arc flash to a big degree relative to instantaneous tripping of molded case circuit breakers and power circuit breakers .

 

[Ian1981]

Here are some printed tables from Hager and MK which may help with regards to the requirements of the cpc at different fault levels using class 3 current limiting mcb’s.